The higher the better and yes 55-65 is good. It really depends on game and gpu settings as well. The higher the settings and resolution the lower the fps you'll get.
That would be highly recommended for your system as you only have 512 at the moment. However if you have the FPS consistantly above 30 FPS then you probably won't notice any difference between that and 60 FPS. The reason is that the human eye can only see about 25 FPS give or take a little depending on the person. So anything over that you don't even see...
you'll be able to tell when the framerate goes below "good" lol
23 would be playable... but barely.
the higher the fps the better, up to a certain point of course. wouldnt want your brain turning into pudding.